Three basic types of coil compression springs are known in the industry. An open end spring consists of a wire coil which generally follows a single helix angle to the end of the wire. An unground, closed end spring has an end which touches the last coil of the spring. In a ground, closed end spring, the tip of the final coil is shaped by grinding such that when the end surface of the tip touches the last active coil of the spring, a flat upper surface is produced. Most standard automotive springs are open end springs as they are relatively inexpensive to produce. In contrast, most high-performance springs used in racecars are ground, closed end springs.
As a fixed plane of contact load is applied to compress a coil spring, as is typical in the majority of springing systems, the spring reactive force is not distributed evenly across the face of the spring. Where this load concentration occurs on the spring varies with the type of spring used. For example, in an open end spring the load is concentrated between the end of the spring and the point at which the load ceases contact with the spring. As the load is increased, this point moves away from the end tip of the spring. In unground closed end springs, the load is concentrated primarily near the end tip. In ground, closed end springs, the load concentration is generally at the first point of active coil contact with the surface of the fixed plane of contact load. The consequences of this uneven loading are illustrated in lateral or offset spring reactive forces such as in vehicle suspension systems. In general, a racing vehicle suspension system is provided with a helical compression spring designed to provide a coil axis that coincides with the direction and axis of the applied load. The most common system employs a spring that is fitted around the body of a shock absorber such that the central axis of the spring substantially coincides with the central axis of the shock absorber. The resultant offset of the spring reactive loads to the fixed-plane-of-contact vehicle loads produce a bending moment in the shock absorber, increasing internal frictions within the shock absorber that degrade the vehicles performance. In a strut-type suspension system, such as is common in street vehicles, a shock absorber is employed as a strut for positioning the vehicle's wheels. If there is a displacement between the load axis and the spring reactive force axis, a bending moment is exerted on the strut, degrading its ability to act smoothly in absorbing road surface inputs. This lateral force may prevent the piston from sliding smoothly in the guide to act as a shock absorber. For this reason, strut systems commonly employ springs that are purposely offset from the axis of the strut such that the spring reactive forces substantially coincides with the vehicle load axis.
In order for the reactive force developed within a spring to remain substantially at the spring's natural center axis, the applied load and reactive forces must be allowed to spread themselves equally over the full face of the spring end coil. It can only do so if the contact plane thru which the load is applied is allowed to pivot, or tilt, as demanded by the twisting stiffness of the spring end coil. If the load is applied through a fixed-planar surface, the load and therefore the reactive force will always be concentrated away from the spring natural center axis.
This problem is illustrated in FIGS. 1-2. A traditional closed-end coil spring 200 having a load-bearing platform 210 at one end is shown in an unloaded state in FIG. 1 disposed against a base 212. In the unloaded state, the first side of the spring 202 is substantially equal in height to the second side of the spring 204. In this example, the point of first contact 206 between the spring 200 and the platform 210 is on the second side of the spring 204.
When a load 220 is applied to the spring 200, the spring is compressed as shown in FIG. 2. As the load is applied, the load is initially resisted at the first point of active coil contact 206, as the load settles and stays in full contact with platform 210, the platform deflects downward on the first side of the spring 202, pivoting around the center axis of the wire at the first point of contact 206. The degree of pivot or tilt is dependent on the twisting stiffness of the wire. As a result, the first side of the spring 202 is compressed to a greater degree than the second side of the spring 204. When a load is applied thru a fixed plane of contact, the resultant spring reactive forces are offset away from the spring natural centerline, resulting in a bending moment applied to the spring assembly. This bending moment is usually undesirable and may result in unanticipated or degraded performance or premature wear of the final spring assembly. Typically this problem has been compensated for by using larger and heavier springs in the context of vehicle suspension systems.
Accordingly, there is a need for a device which assists in centering the reactive loads in a coil spring, preferably allowing the load to be concentrated at the centerline of the spring. In the context of vehicle suspension systems, preferably such a device is lighter and more efficient than current devices. Prior attempts to solve these problems have been unsuccessful. The present invention addresses these concerns.